CSE 351 x86 Calling Conventions, Control Flow, & Lab 2 April 23, 2015

GDB Part II: Using GDB with C files GDB Cheat Sheet dbnotes-x86-64.pdf dbnotes-x86-64.pdf Go on: Scroll down to “Review with two examples” Download files and follow steps in GDB commands linked

Calling Conventions Review This class uses x86-64 conventions, which differ from standard x86 conventions The first six arguments are passed in registers: %rdi, %rsi, %rdx, %rcx, %r8, %r9 Any additional arguments go on the stack Why? Faster!

Caller- vs. Callee-Saved Registers Saving registers Accomplished via the push command Restored using pop Callee-saved registers Must be saved by a function before changing its value, and then restored before returning %rbx, %rbp, %r12-%r15 Caller-saved registers Must be saved by a function before calling any other subroutines if it wishes to preserve the value All other registers (including all the parameter-passing registers)

Callee/Caller Explained Why not have all registers caller, or all registers callee? We want to minimize the number of pushes/pops (they’re slow!) Think of callee registers are non-volatile You must save them to use them, BUT… You can be guaranteed that when you call a function, it will have the same value when it returns Good for values that must be preserved for a long time, across many calls Think of caller registers as volatile You don’t have to save them in order to use them, SO… If you ever have values that you need before a function call but don’t need afterward (i.e. arguments) then they should be in caller-saved registers This way, no values are pushed onto the stack unnecessarily

Control Flow 1-bit condition code registers [CF, SF, ZF, OF] Set as side effect by arithmetic instructions or by cmp, test CF – Carry Flag Set if addition causes a carry out of the most significant (leftmost) bit. SF – Sign Flag Set if the result had its most significant bit set (negative in two’s complement) ZF – Zero Flag Set if the result was zero OF – Overflow Flag If the addition with the sign bits off yields a result number with the sign bit on or vice versa

Control Flow Examples x86: test %rax, %rax je Result: ; set ZF to 1 if rax == 0 Jumps to if rax == 0 ; jump if ZF == 1 cmp %rax, %rbx jg (hint: jg checks if ZF = 0 and SF = OF) rax and rbx are interpreted as signed then compared, if rbx > rax we jump to cmp %rax, %rbx xor %rbx, %rbx js (hint: js checks if MSB of result = 1) Never jumps to

Lab 2 Questions/Concerns about Lab 2